A process similar to that as defined above is described in document EP-A-0451456. In this known process, all of the calcine produced in the operation (a) is leached in the operation (b) and all of the ferrite produced in the operation (b) is leached in the operation (c), while employing a concentrate:ferrite ratio such that approximately 15 to 20% of the trivalent iron, required for the oxidation according to reaction (I) of the reactive sulphur present in the concentrate, originates from the leaching of the ferrite according to the reaction EQU ZnO.Fe.sub.2 O.sub.3 +4H.sub.2 SO.sub.4 =Fe.sub.2 (SO.sub.4).sub.3 +ZnSO.sub.4 +4H.sub.2 O (II)
The remainder of the trivalent iron required for the oxidation of the reactive sulphur is obtained by the reaction EQU 2FeSO.sub.4 +H.sub.2 SO.sub.4 +0.50.sub.2 =Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O (III)
It is proposed to work in (c) in such a manner that the leachate which is rich in zinc and in iron has a sulphuric acid content of 10-25 g/l and an Fe.sup.3+ content of less than 10 g/l, which is apparently unobtainable in a single leaching stage. This is why the leaching in (c) is carried out in two stages.
In the first stage, the zinc ferrite and a leaching residue which is rich in zinc, produced in the second stage, are treated with the solution of acid returning from electrolysis so as to produce a primary leachate containing 50-90 g/l of H.sub.2 SO.sub.4 and the leaching residue which is depleted in zinc, which are separated. No oxygen is employed in this first leaching stage, this being to make it possible to use in this stage simpler types of reactors than in the second stage. The involvement of reaction (III) is therefore not brought about in the first stage.
In the second stage, the concentrates are treated with the said primary leachate in the presence of finely dispersed oxygen so as to produce, by reactions (I) and (III), a leachate which is rich in zinc and in iron and a leaching residue which is rich in zinc. At the end of this second stage, the operation (d) is performed by adding a small quantity of fresh concentrate to the leaching pulp so as to convert ferric sulphate into ferrous sulphate by reaction (I); the operation (d) is therefore incorporated into the operation (c) and, as a result of the second leaching stage, there is obtained the leaching residue which is rich in zinc, which is separated and recycled into the first stage, and a leachate which is rich in zinc and in iron, which is already conditioned.
The operation (e) is performed by adding more concentrate to the conditioned solution and by then precipitating the iron in haematite form by oxidation in an autoclave.
This produces, on the one hand, the said solution which is rich in zinc and depleted in iron and, on the other hand, a precipitate of haematite containing a small quantity of elemental sulphur and of sulphides. This sulphur and these sulphides are subsequently separated from the haematite by flotation. The reason why the concentrate is used in the operation (e) is not given. A possible explanation could be that the acidity of the conditioned solution is too high to permit a suitable precipitation of the iron and that, because of this, concentrate is added as neutralizing agent (reactions (I) and (II)).
This known process therefore requires leaching in two stages and, since the work is carried out with a concentrates:ferrite ratio such that approximately 15 to 20% of the trivalent iron, required for the oxidation according to reaction (I) of the reactive sulphur present in the concentrate, originates from the leaching of the ferrite in accordance with reaction (II) and that oxygen is not employed in the first leaching stage, it is necessary to oxidize approximately 80 to 85% of the reactive sulphur in the second leaching stage using trivalent iron obtained by reaction (III), when a zinc leaching yield close on 100% is aimed at.
However, the Applicant has found that in these conditions the second leaching stage takes place very slowly, and this obviously constitutes a serious disadvantage. Another disadvantage of this known process lies in the fact that the operation (c) is not easy to control because the leaching yield is determined solely by the ratio of the reactive sulphur to the zinc ferrite which are introduced into the first leaching stage and because the system reacts very slowly to corrections which are made to this ratio.
Furthermore, the use of this known process in existing hydrometallurgical zinc plants would almost always entail a considerable investment for purchasing the autoclaves required for the operation (e). In fact, to the Applicant's knowledge, there are only two plants in the world which make haematite and which are therefore already equipped with such autoclaves; all the others make jarosite or goethite in atmospheric conditions and are therefore not endowed with such autoclaves.
Moreover, as in this known process all of the ferrite produced in the operation (b) is leached in (c) while employing a concentrate:ferrite ratio such that approximately 15 to 20% of the trivalent iron, required for the oxidation of the reactive sulphur, originates from the leaching of the ferrite, the installation of this process in an existing plant, the roasting capacity of which would quite logically be maintained at the existing level, would have to result at once in approximately doubling the plant capacity. However, an increase in the capacity of an existing plant which is as substantial as this all at once, which will be unavoidably accompanied with substantial investments, will not often be opportune. The process therefore lacks some degree of flexibility.
What is more, this known process produces a haematite which is soiled with sulphur and sulphides.